This application relates generally to the field of electrical circuits and in particular to the manufacture of multi-layer printed circuit boards.
Some components, typically high power components, are unsuitable to be incorporated in a multi-layer printed circuit board or mounted on the surface of a multi-layer printed circuit board with conventional automated assembly techniques. Typically, components having large heat sinks and/or utilizing attachment to an external heat sink or heat spreader, such as power transistor, and other flanged parts are especially problematic to automated assembly technique, such as pick and place. Typical problem components may include, but are not limited to, high-power transistors as used in a power supply, and high-power transistors and high-power attenuators typically used in radio frequency (RF) or microwave circuits. The heat sinks of these parts often serve as circuit ground connections.
A typical method of mounting high power components is to cut a hole large enough to clear the component through the multi-layer printed circuit board and then attach the component to an external heat sink. For example, if the component is a flanged high-power radio frequency (xe2x80x9cRFxe2x80x9d) transistor, the hole is made large enough for the flange to pass through. A thermal and electrical connection is then typically formed between the flange and a heat sink, typically by using mounting hardware, or solder. Heat sinks are typically constructed as a pallet formed of a metal such as copper or aluminum that possesses good thermal conductivity.
As will be appreciated by those skilled in the art, it is difficult to satisfactorily solder a flanged part to a large mass of metal, such as a heat sink. With the increased use of high thermal conductivity heat sinks such as copper coated graphite composites, the problem becomes more acute. Mechanical attachment with thermal grease, screws and the like is an alternative method of attachment that is usually undesirable due to the additional manufacturing steps and additional hardware required.
In the construction of radio frequency (RF), and microwave circuits, a low-impedance connection is often desirable between a bottom side ground layer of a multi-layer printed circuit board (multi-layer PCB), the flanged part, and the pallet especially at the edges of a clearance hole made in the multi-layer PCB. A good connection at the edges of the hole is desirable because an RF return current travels by transitions from the PCB to the flange of the transistor through the heat sink. This connection is typically made by assembling the multi-layer PCB to a copper or aluminum pallet using screws or a conductive medium such as solder or conductive epoxy. The flanged part is similarly attached to the heat sink. Thus, and the heat sink serves to couple the PCB electrical ground to the ground of the flanged part.
All of the foregoing methods have associated manufacturing difficulties, poor electrical performance, and correspondingly high manufacturing costs. Accordingly, there is a need for a lower-cost method of assembling multi-layer PCBs having components heat sinks, or pallets, and components (such as flanged RF power transistor) mounted to the board, and to the pallet that preserve a low impedance electrical ground connection to and from the component.
According to one aspect of the invention a method of mounting a flanged component to a printed wiring board that has been bonded to a pallet, is provided. The method of mounting tends to provide RF grounding of the flanged component to the pallet, and from ground circuitry on the printed wiring board to the pallet. The pallet is typically a metal heat sink, such as a copper block. The steps of the method of mounting comprise fabricating a printed wiring board having a clearance hole cut through it to accommodate the flange mounted component. The walls of the clearance hole are plated with a conductor. The plating around the top of the hole is removed, such that the plating remaining on the walls is coupled to an electrical ground connection available from the printed wiring board. The bottom of the printed wiring board is glued or bonded to the metal pallet. Solder, and then the flanged component are disposed in the hole. The solder is melted, flowing to the walls and onto the pallet. The solder connection formed electrically couples the printed wiring board ground to the flange and the pallet.
According to another aspect of the invention a method of mounting a flanged component to a printed wiring board that has been bonded to a pallet, is provided. The method of mounting tends to provide RF grounding of the flanged component to the pallet, and from ground circuitry on the printed wiring board to the pallet. The pallet is typically a metal heat sink, such as a copper block. The steps of the method of mounting comprise fabricating a printed wiring board having a well disposed in it of a size sufficient to accommodate the flange mounted component. The well is plated with a conductor, such that metal is disposed upon the walls of the well. A hole is made such that the bottom of the well is substantially removed. A hole in the printed wiring board is thus formed in which the plating around the top of the hole is removed, such that the plating remaining on the walls is coupled to an electrical ground connection available from the printed wiring board. The bottom of the printed wiring board is glued or bonded to the metal pallet. Solder, and then the flanged component are disposed in the hole. The solder is melted, flowing to the walls and onto the pallet. The solder connection formed electrically couples the printed wiring board ground to the flange and the pallet.